Alzheimer's disease (AD) is the most prevalent form of dementia. It is a neurodegenerative disorder that is associated (though not exclusively) with aging. The disorder is clinically characterized by a progressive loss of memory, cognition, reasoning and judgment that leads to an extreme mental deterioration and ultimately death. The disorder is pathologically characterized by the deposition of extracellular plaques and the presence of neurofibrillary tangles. These plaques are considered to play an important role in the pathogenesis of the disease.
These plaques mainly comprise of fibrillar aggregates of β-amyloid peptide (Aβ), which are products of the amyloid precursor protein (APP), a 695 amino-acid protein. APP is initially processed by β-secretase forming a secreted peptide and a membrane bound C99 fragment. The C99 fragment is subsequently processed by the proteolytic activity of γ-secretase. Multiple sites of proteolysis on the C99 fragment lead to the production of a range of smaller peptides (Aβ 37-42 amino acids). N-terminal truncations can also be found e.g. Aβ (4-42, 11-42) for convenience Aβ40 and Aβ42 as used herein incorporates these N-terminal truncated peptides. Upon secretion, the Aβ peptides initially form soluble aggregates which ultimately lead to the formation of insoluble deposits and plaques. Aβ42 is believed to be the most neurotoxic, the shorter peptides have less propensity to aggregate and form plaques. The Aβ plaques in the brain are also associated with cerebral amyloid angiopathy, hereditary cerebral hemorrhage with amyloidosis, multi infarct dementia, dementia pugilistisca, inclusion body myositis and Down's Syndrome.
γ-secretase is an association of four proteins: Aph1, Nicastrin, Presenillin and Pen-2 (review De Strooper 2003, Neuron 38, 9). Aβ42 is selectively increased in patients carrying particular mutations in one of these components, presenilin. These mutations are correlated with early onset a familial AD. Inhibition of γ-secretase resulting in the lowering of Aβ42 is a desirable activity for the pharmaceutical community and numerous inhibitors have been found, e.g., Thompson et al (Bio. Org. and Med. Chem. Letters 2006, 16, 2357-63), Shaw et al (Bio. Org. and Med. Chem. Letters 2006, 17, 511-16) and Asberom et al (Bio. Org. and Med. Chem. Letters 2007, 15, 2219-2223). Inhibition of γ-secretase though is not without side-effects, some of which are due to the γ-secretase complex processing substrates other than C99, for e.g. Notch. A more desirable approach is to modulate the proteolytic activity of the γ-secretase complex in a manner that lowers Aβ42 in favor of shorter peptides without significantly affecting the activity of γ-secretase on substrates such as Notch.
Compounds that have shown modulation of γ-secretase include certain non-steroidal, anti-inflammatory drugs (NSAIDs), for example Flurbiprofen, (Stock et al Bio. Org. and Med. Chem. Letters 2006, 16, 2219-2223). Other publications that disclose agents said to reduce Aβ42 through the modulation of γ-secretase include: WO 04/074232, WO 05/054193, Perreto et al Journal of Medicinal Chemistry 2005, 48 5705-20, WO05/108362, WO 06/008558, WO 06/021441, WO 06/041874, WO 06/045554, WO04110350, WO 06/043964, WO 05/115990, EP1847524, WO 07/116,228, WO 07/110,667, WO 07/124,394, EP184752, EP 01849762, WO 07/125,364.